Corrosion-resistant nickel-molybdenum alloys

ABSTRACT

Nickel-molybdenum alloys having increased corrosion resistance and resistance to impact at room temperature and below resulting from a critical control of the amounts of carbon, silicon, vanadium, boron and zirconium without creating vanadium stabilization. The carbon and silicon are maintained in negligible amounts and the vanadium, boron and zirconium are closely controlled.

Ecer 5] Mar. 14, 1972 [54] CORROSION-RESISTANT NICKEL- MOLYBDENUM ALLOYS [72] Inventor: Gunes M. Ecer, Pittsburgh, Pa.

[73] Assignee: Cyclops Corporation, Universal Cyclops Specialty Steel Div., Pittsburgh, Pa. 57

[22] Filed: May 25,1970

[21] Appl.No.: 40,460

2,959,480 11/1960 Flint ..75/170 Primary Examiner-Richard 0. Dean Attorney-Webb, Burden, Robinson & Webb ABSTRACT Nickel-molybdenum alloys having increased corrosion resistance and resistance to impact at room temperature and below resulting from a critical control of the amounts of carbon silicon vanadium boron and zirconium without creating ..7 7 I i vanadium stabilization. The carbon and silicon are maintained [58 Field of Search ..75/170, 171; 148/32, 32.5 in negligible amounts and the vanadium boron and Zimnium are closely controlled.

R [56] eferences Cited 2 Claims, No Drawings UNITED STATES PATENTS r W 1,836,31712/1931 Franks ..75/171 CORROSION-RESISTANT NICKEL-MOLYBDENUM ALLOYS BACKGROUND Nickel-molybdenum alloys are known to be especially suited for construction of equipment for chemical plants because of their high corrosion resistance to various acids and their good physical properties at room temperature and above. These alloys are known to have excellent resistance to the corrosive attack of hydrochloric acid, phosphoric acid, sulfuric acid and all types of organic acids.

The usual techniques for fabricating chemical plant equipment, such as tanks and pipes involves welding. Welding, which subjects portions of the alloys to high temperatures results in reduced intergranular corrosion resistance near the welded joints. The poor corrosion resistance after welding may be overcome in nickel-molybdenum alloys by a post-weld heat treatment comprising heating the fabricated alloys at a temperature in the range of 900 to l,175 C. for about 2 to 72 hours. However, this treatment introduces practical problems, which are obvious to metallurgists, for example, scaling and distortion, due to expansion stresses on heating and cooling. This technique for restoring resistance to intergranular corrosion through postweld heat treatment has not enjoyed wide popularity.

Another technique for avoiding intergranular corrosion in nickel-molybdenum alloys after welding has been vanadium stabilization." This technique is described, for example, in US Pat. No. 2,959,480, issued to G. N. Flint. Basically, vanadium ranging in amounts from 1.1 to 2.3percent is incorporated in the alloy, which in some manner not entirely understood, prevents the welded nickel-molybdenum alloy from becoming vulnerable to intergranular corrosion. Vanadium stabilized alloys have drawbacks: Because of their compositions, they have not developed their ultimate corrosion resistance and notch toughness, especially at room temperature and below.

In recent years the growth of cryogenic applications, such as the manufacture of liquified gases, has created a demand for chemical plant equipment that has the requisite mechanical strength at cryogenic temperatures defined herein as below C. Most alloys are very brittle at these temperatures and, therefore. unsuitable for these applications.

It has now been discovered that nickel-molybdenum alloys with the improved corrosion resistance and very good low temperature impact strength, as measured by notch ductility tests, can be obtained by close control of the amounts of carbon, silicon, vanadium, boron and zirconium present in nickel-molybdenum alloys but without vanadium stabilization as already described. Alloys according to this invention are advantageous, because they are resistant to intergranular corrosion when welded and do not need a postwelding solution treatment. Further, these alloys have good tensile properties, and high impact strength at room temperatures and cryogenic temperatures and good workability. Alloys according to this invention have corrosion resistance superior to vanadium stabilized alloys before and after welding.

THE INVENTION Briefly, according to this invention, there is provided an alloy comprising molybdenum from 20 to 40 percent by weight, iron up to 10 percent, cobalt up to 4 percent, chromium up to 5 percent, manganese up to 2 percent, phosphorous up to 0.03 percent, sulphur up to 0.03 percent, carbon up to 0.1 percent, silicon up to 0.1 percent, vanadium from 0.1 to 1.0 percent, boron from 0.001 to 0.035 percent, zirconium from 0.01 to 1 percent, and the remainder nickel plus incidental impurities.

According to a preferred embodiment of this invention, an alloy is provided comprising 26 to 32 percent by weight molybdenum, up to 7 percent iron, up to 2.5 percent cobalt, up to 1 percent chromium, up to 1 percent manganese, up to 0.025 percent phosphorous, up to 0.03 percent sulphur, up to 0.03 percent carbon, up to 0.03 percent silicon, from 0.2 to 0.8 percent vanadium, from 0.001 to 0.02 percent boron, from 0.01 to 0.1 percent zirconium, and the remainder nickel and incidental impurities.

EXAMPLE A 3,000 pound heat of an alloy, according to this invention, hereafter referred to as alloy A, was prepared. To insure a siliconand carbon-free composition, the heat was melted under vacuum using raw materials relatively free of these elements. While in this example the melt was prepared using vacuum techniques, with other proper melting techniques the improvements mentioned herein would be attainable. The chemical analysis of the ingots cast from the melt was as follows: carbon 0.002 percent, manganese 0.460 percent, silicon less than 0.010 percent, sulphur 0.010 percent. phosphorous 0.002 percent, molybdenum 26.900 percent, iron 5.250 percent, cobalt 1.010 percent, vanadium 0.240 percent, chromium 0.140 percent, boron 0.0025 percent, zirconium 0.029 percent, the balance nickel, except for incidental trace impurities including magnesium, calcium and oxygen.

The ingots cast from the melt had 1 l-inch X 15-inch cross sections. These ingots were hot forged (1,900 to 2,l75 F.) into 3.4-inch X 1 l-inch slabs. The slabs were hot-rolled to A- inch-thick plates, annealed and pickled. The good hot workability of alloy A, according to this invention, enabled it to be broken down and rolled using well-known techniques.

COMPARATIVE TESTING The properties, both physical and chemical, of the alloy A, were ascertained and compared with the properties of two related nickel-molybdenum alloys having the chemical compositions set forth in the following table:

Corrosion testing samples of alloy A and comparative alloy B were prepared from solution treated (2,165 F. 20 minutes water quench) plates with dimensions of /a-inch X 7'7-inches X 6 inches. Some samples were tested as is while others were TIG welded using pure tungsten electrodes under an argon atmosphere. One sample plate of each alloy was given a single pass of the welding electrode and another sample of each alloy was given three passes of the electrode. The unwelded and welded plates were then sectioned into A-inch 15-inch X Xi-inch sections. The sections of the welded plates included the weld bead. Each specimen was surface ground on all faces, including the weld bead, ultrasonically degreased in freon fluorocarbon and cleaned in a 10 percent nitric acid solution.

The dimensions of the corrosion samples were measured to the nearest thousandth of an inch. They were weighed in grams to the fourth decimal place. The samples were tested for corrosion in boiling 20 percent hydrochloric acid, boiling Alloy A and the vanadium stabilized alloy B have comparable tensile properties up to at least l,200 F. Comparative room temperature tensile data is given in the following table:

85 percent phosphoric acid and boiling 20 percent formic acid.

After corrosion testing, from the density of the samples, the

TABLE III.TENSILE DATA (ROOM TEMPERATURE TESTS) Percent Ultimate 0.2% ofitensile set yield Elon- Reducstrength strength gation tion of Grade and condition (p.s.i.) (p.s.i.) in 1" area Alloy A, 0.6 inch plate:

2,100 F., 15 min., WQ (transverse) 127, 200 54,800 68. 6 77. 2,150 F., min., WQ:

LongitudinaL. 125, 500 52, 900 69. 3 T8. 7 Transverse 124, 800 52, 400 60. 1 77. 8 2,200 F., 15 min., Q (transverse) 125,000 54,100 72. 2 77. 7 Alloy B:

2,165 F., WQ:

0.375 inch plate 127, 200 55,500 66, 0 0.750 inch plate 123, 400 59, 800 57. H

B 'gslgl ulcgsfipbtoincd from Union Carbidc's preliminary data publication llastclloy Alloy impact tests using the Charpy (single beam) impact device were made on alloy A and alloy B at room temperature and at cryogenic temperatures. Standard size V-notch Charpy im- TABLE II.CORROSION PROPERTIES Corrosion rates Acid Sample I.p.m. I.p.y.

% hydrochloric acid (boiling) Alloy A, inch plate (2,165 F.20 min.-WQ) 0. 0032 0. 0380 Alloy A, .08 inch sheet (2,125 F-20 min.-WQ) 0. 0019 0. 0229 Alloy 13, $4 inch plato (2,165 F.20 min.-WQ) 0. 0036 0. 0447 Alloy A, M inch plate, its-welded (one pass) 0 0007 0. 0082 Alloy B, inch plate, as-welded (one pass) 0.0010 0.0160 Alloy A, inch plate, as-welded (three passes) 0. 0032 0. 0389 Alloy 13, 54 inch plate, as-welded (three passes) 0. 0043 0. 0518 85% phosphoric acid (boiling) Alloy A, 54 inch plate (2,165 F.20 min.-WQ) 0. 00030 0. 0040 Alloy B, M inch plate (2,165 F.20 min.-WQ) 0. 00067 0. 00746 20% formic acid (boiling) Alloy A, 54 inch plate (2,165 F.20 Inin.-WQ) 0. 0000 0.0116 Alloy B, )4 inch plate (2,165 F.20 min.-WQ) 0. 0016 0. 0175 pact specimen were prepared from solution treated 0.6-inchthick plates of the new alloy A. More notched and unnotched subsized specimens (0.225 inches X 0.225 inches X 2.165 inches with V-notch depth 0.050 inches) were prepared from A-inch-thick solution treated alloy A and alloy B. These Table ll established that alloy A, according to this invention, has better corrosion resistance than the vanadium stabilized alloy B in the solution treated or after welded condition. Alloy A is also superior to alloy C (not vanadium stabilized) in the solution treated condition according to published data for the comparative alloy, which gives the corrosion rate in 85 specimens were tested at room temperature, at l20 F. and percent phosphoric acid as 0.028 I,P.Y. (Union Carbide 320 F, The results of these tests are given in table IVA and Technical Brochure, Hastelloy-Corrosion Resistant Alloys table IVB.

TABLE IVA.IMPACT TESTING-STANDARD SIZE SAMPLES Charpy V-noteli Test temimpact Fraction Lateral perature strength ductile contraction Grade and condition F.) (ft.lbs.) (percent) (percent) Comment Alloy A, 0.6 inch plate an- Room-.." 240 100 26, 0 Did not giggled 2,150 F., 15 min., break. -120 240 100 25. a Do. 320 240 100 24. 6 Do. Alloy B, 0.750 inch plate Room 78 annealed 2,165 F., WQ. Alloy C ..do 58-62 *Data obtained from Union Carbides brochures, supra.

May, 1967). Furthermore, alloy A possesses the same high TABLE IVBPIMPACT TESTING SUBSIZE SAMPLES corrosion resistance even in as-welded condition, whereas alloy Cs susceptibility to intergranular corrosion after welding lz gfig g is well known. Grade and condition Specimen F.) Fracture mode The optimum recommended annealing temperature for Alloy A, 0.250 inch plate Unnotched Room..- Did Not}? rack.

alloy B is 2,] F., whereas for alloy A the optimum tempera- 65 (2,165F.20 min.-WQ). Notched do ture has been found to be 2,125 F. Table ll directly compares d 0 58' the corrosion resistance of alloy A with alloy B after a 2,165 20 Do:

F. annealing treatment. Data are also set forth in table ll show- Notched r ing the corrosion resistance of alloy A after annealing at its op- Alloy B, 0.250 inch plate Unnotched.-. Room... Did Not Crack,

timum annealing temperature. Thus, alloy A is considerably (ammamm i jgtfi superior to alloy B in corrosion resistance when each alloy is Do.

given its optimum annealing treatment. 32, Crack The lower optimum annealing temperature for alloy A Notched -320 Brittle tracfacilitates heattreating procedures. The lower temperature minimizes warpage and surface oxidization (scaling). Also, *Speeimenswere toothin to break completely.

*Notehes opened up slightly in a ductile manner. *Cracked at notches approximately of specimen thickness in a brittle manner.

the lower temperature is more easily achieved in most industrial shops.

Table WA and table lVB establish that alloy A, according to this invention, has unusually superior impact properties, both at room temperature and down to 320 F.

The scientific basis for the improved corrosion resistance and impact strength of nickel-molybdenum alloys, according to this invention, is not entirely understood. However, it is believed that substantial elimination of carbon and silicon and the presence of boron, zirconium, and vanadium in controlled amounts contribute to the homogeneity of the alloy by preventing grain boundary precipitation ofdetrimental secondary phases and by establishing a more stable chemical and electronic balance which prevents migration of deleterious trace elements to the grain boundaries. The beneficial structural stability is achieved without the aid of vanadium stabilization.

Having thus defined the invention with the detail and particularity required by the Patent Statutes what is desired protected by Letters Patent is set forth in the following claims.

lclaim:

1. An alloy having improved corrosion resistance and impact strength at room temperature and below consisting essentially of molybdenum from 20 to 40 percent by weight, iron up to 10 percent, cobalt up to 4 percent, chromium up to 5 percent, manganese up to 2 percent, phosphorous up to 0.03 percent, sulphur up to 0.03 percent, carbon up to 0.l percent, silicon up to 0.] percent, vanadium from 0.] to l percent, boron from 0.00l to 0.035 percent, zirconium from 0.0l to l percent, and the remainder nickel plus incidental impuritiesv 2. An alloy having improved corrosion resistance properties and improved impact properties at room temperature and below consisting essentially of 26 to 32 percent molybdenum by weight, up to 7 percent iron, up to 2.5 percent cobalt, up to 1 percent chromium, up to l percent manganese, up to 0.025 percent phosphorous, up to 0.03 percent sulphur, up to 0.03 percent carbon, up to 0.03 percent silicon, from 0.2 to 0.8 percent vanadium from 0.001 to 0.02 percent boron, and from 0.01 to 0.1 percent zirconium, and the remainder nickel and incidental impurities. 

2. An alloy having improved corrosion resistance properties and improved impact properties at room temperature and below consisting essentially of 26 to 32 percent molybdenum by weight, up to 7 percent iron, up to 2.5 percent cobalt, up to 1 percent chromium, up to 1 percent manganese, up to 0.025 percent phosphorous, up to 0.03 percent sulphur, up to 0.03 percent carbon, up to 0.03 percent silicon, from 0.2 to 0.8 percent vanadium from 0.001 to 0.02 percent boron, and from 0.01 to 0.1 percent zirconium, and the remainder nickel and incidental impurities. 